1) Field of the Invention
This invention pertains to the field of image display systems, and more particularly to image projectors employing liquid crystal light valves.
2) Description of the Related Art
Projection-type image display systems are widely known. Examples of such systems, and in particular systems using liquid crystal light valves, are shown in U.S. Pat. Nos. 5,532,763, 5,786,934, 6,224,215 and 6,234,634 the entirety of each of which is hereby incorporated by reference for all purposes as if fully set forth herein. With reference to the following description, familiarity with conventional features of such systems will be assumed, so that only features bearing on the present invention will be described.
FIG. 1 shows relevant portions of an exemplary liquid crystal display (LCD) projector 100. The LCD projector 100 is a single panel color LCD projector.
The LCD projector 100 comprises in relevant part: a lamp 102; a first integrator 104; a second integrator 105; a polarization conversion system (PCS) 106; a condenser lens 107; first through fourth dichoic color filters 108, 110, 112 and 114; first to third scanning prisms 116, 118 and 120; reflecting mirrors 122 and 124; pre-polarizer 126; polarizing beam splitter 128; LCD 130; analyzer 132; and projection lens 134.
The operation of the LCD projector 100 will now be explained. The LCD device 130 operates as a light valve or modulator and is adapted to receive an incident light and to impress a desired image upon the incident light which is then projected by the projection lens 134 onto a desired display area or screen. To produce a high quality image with a high contrast ratio, the LCD 130 requires that the incident light be substantially linearly polarized in a predefined direction. In the LCD projector 100, the LCD device 130 is preferably a reflective-type LCD device such as a liquid crystal on silicon (LCOS) device. The operation of reflective-type LCD devices is well known, and may be found, for example in U.S. Pat. Nos. 6,023,309 and 6,052,165.
The exemplary LCD projector 100 shown in FIG. 1 employs a scrolling color system. In a scrolling color system, white light is split into constituent red (R), green (G), and blue (B) colored light beams. The colored light beams illuminate the LCD panel as spatially separated stripes of light. The stripes of light scroll across the LCD from top to bottom. As one color scrolls off the bottom of the LCD, it appears immediately again at the top. All three colors (R, G and B) are present on the LCD at all times.
To create the color image, the LCD is effectively addressed at three different places simultaneously. Data for a given color (e.g., green), is written to a row just after the passing of the previous color (e.g., blue). The (green) band of colored light then illuminates the pixel. The panel is addressed at three places and these address rows shift downward in sync with the illumination pattern. A more detailed explanation of the operation of the scrolling color architecture may be found in xe2x80x9cSingle Panel Reflective LCD Projector,xe2x80x9d J. A. Shimizu, Projection Displays V, Proceedings SPIE, Vol. 3634, pp. 197-206, (1999), the entirety of which is hereby incorporated by reference for all purposes as if fully set forth herein.
Turning once again to FIG. 1, the lamp 102 produces a high intensity, generally unpolarized light. The integrator 104 receives the light from the lamp 102 and shapes and homogenizes the distribution of the light to produce a generally rectangular beam. The integrators 104 and 105 are typically each comprised of a lens array. The integrators 104 and 105 produce multiple secondary light source images in a focal plane, F.
The PCS 106 is located in the focal plane F of the integrators and serves to substantially linearly polarize the light incident thereon.
FIG. 2 shows one embodiment of an integrator 105, a PCS 106, and a condenser lens 107 that may be used in the LCD projector 100. The integrator 105 comprises a lens array. The PCS 106 comprises a polarizing beam splitter array 210 and a half wave plate 220. The polarizing beam splitter array 210 includes an alternating arrangement of polarizing beam splitters 212 and mirrors 214.
The PCS 106 is adapted to receive generally unpolarized light and to convert the generally unpolarized incident light into linearly polarized light, xe2x80x9cP.xe2x80x9d The polarized light may then be passed out through the condenser lens 107. In contrast to a standard absorbing polarizer, which passes only roughly 50% of light incident thereon having a desired polarization (e.g., xe2x80x9cPxe2x80x9d), and which absorbs roughly 50% of the light incident thereon having an undesired polarization (e.g., xe2x80x9cSxe2x80x9d), the PCS 106 typically passes a much higher percentage of incident light than a polarizer (e.g., 80% of incident light) and absorbs a small fraction of the light energy typically absorbed in an absorbing polarizer. Thus, the PCS 106 may be used to polarize light in high intensity applications (e.g., greater than about several hundred lumens) where a standard absorbing polarizer cannot be used because the absorbed light energy would destroy it. On the other hand, the extinction ratio (Tp/Ts) for the PCS 106 is typically substantially lower than that for an absorbing polarizer, meaning the output light has a lower degree of polarization. An extinction ratio for a typical PCS 106 is less than 10 (e.g., 8.25).
In addition to polarizing the received light beam, the combination of polarizing beam splitter 212, adjacent mirror 214, and half wave plate 220 in the PCS 106 roughly doubles the etendue, or optical size, of the light beam. That is, the PCS 106 roughly doubles the etendue of an illumination system into which it is incorporated.
A more detailed explanation of the operation of the PCS 106 may be found in xe2x80x9cUltra-High-Efficiency LC Projector Using a Polarized Light Illuminating System,xe2x80x9d Y. Itoh et al., 1997 Society of Information Display Digest, pp. 993-996 (1997), the entirety of which is hereby incorporated by reference for all purposes as if fully set forth herein.
Turning again to FIG. 1, the substantially polarized light from the PCS 106 is passed through the dichoic color filters 108 and 110 to separate the polarized light into a plurality of different colored light beams (e.g., three, labeled xe2x80x9cR,xe2x80x9d xe2x80x9cG,xe2x80x9d and xe2x80x9cBxe2x80x9d). Each of the colored light beams, red, green and blue, is provided to a corresponding rotating prism 116, 118, or 120, for scanning the color beam onto the LCD 130 in the scrolling color system. The dichoic color filters 112 and 114 direct the plurality of colored light beams to the pre-polarizer 126.
The pre-polarizer 126 increases the extinction ratio of the colored light beams incident thereon to provide very highly polarized light to the LCD 130. The pre-polarizer has in its plane both an absorbing direction and a non-absorbing direction, the absorbing direction being perpendicular to the non-absorbing direction. Light having a desired polarization xe2x80x9cPxe2x80x9d along the non-absorbing direction of the pre-polarizer 126 passes through the pre-polarizer 126 to the polarizing beam splitter 128, and the remaining light is generally absorbed by the polarizer. The polarizer, being a non-ideal device, has a finite extinction ratio, and thus a small amount of the colored light beams having the undesired xe2x80x9cSxe2x80x9d polarization passes through the device.
The substantially polarized colored light beams from the pre-polarizer 126 are supplied to the polarizing beam splitter 128. The polarizing beam splitter 128 directs the substantially polarized colored light beams to the LCD device 130. Meanwhile, a small remnant of the colored light beams having the undesired xe2x80x9cSxe2x80x9d polarization is directed away from the LCD device 130 by the beam splitter 128 (e.g., passes through the device).
As briefly described above, the LCD device 130 imparts desired image information on the substantially polarized colored beams by modulating the polarization of the incident light and reflecting the imaged light back to the polarizing beam splitter 128.
The polarizing beam splitter 128 directs the imaged light from the LCD device 130 having a desired polarization xe2x80x9cSxe2x80x9d to the analyzer 132, while directing reflected light from the LCD device 130 having an undesired polarization xe2x80x9cPxe2x80x9d away from the analyzer 132. The analyzer 132 further polarizes the imaged light incident thereon and provides the imaged light to the projection lens 134.
Finally, the projection lens 134 magnifies and projects the imaged light onto a desired display area or screen.
While this related art LCD projector is satisfactory for projecting an image, it has several limitations. The PCS is very costly and complicated to manufacture. Moreover, as explained above, the PCS doubles the etendue of the illumination system, roughly doubling the size of the LCD that is needed to capture the light beam. The PCS gets very hot and requires cooling, decreasing overall system durability. The pre-polarizer also requires active cooling to prolong its lifetime, further affecting system durability. Also, the PCS is not suitable for higher power light sources (e.g., much greater than 1000 watts) used for high intensity projectors (e.g., greater than 1000 lumens) as the amount of light energy absorbed by the PCS will cause the device to fail.
Accordingly, it would be desirable to provide an image display system suitable for high power projectors, which is more durable, which has less stringent cooling requirements, and preferably which includes less complicated and costly polarizing elements. Other and further objects and advantages will appear hereinafter.
Therefore, in one aspect, the invention provides an image display system suitable for high intensity applications.
Accordingly, an image display system is provided having: an image display system, comprising: a light source emitting substantially unpolarized light; means for separating the substantially unpolarized light into a plurality of colored light beams; a polarizing element adapted to receive at least one of the colored light beams, to direct in a first direction a first portion of the one colored light beam having substantially a first polarization, and to direct in a second direction a second portion of the one colored light beam having substantially a second polarization; an imager adapted to impart image information on at least the one colored light beam; a projection lens adapted to project imaged light; and a polarizing beam splitter adapted to direct the first portion of the one colored light beam from the polarizing element to the imager, and to direct the imaged light from the imager to the projection lens.